Once a week is my minimum, but lately I've been playing about three times a week. Some flowers die and some wilt, but this is why I spent years breeding gold roses. Gold roses don't wilt and die (unless I kill them accidentally, which happens sometimes).
If you play 2-3 times a week and water any wilted flowers, you'll probably lose a few, but not a lot. This time of year is good for flowers though, because it's been snowing a lot lately (and snow waters the flowers). Any that means none will be wilted on the day after it snows/rains.